Crystal oscillator circuit



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- Fig.5.

1945- R. DOWNEY 2,369,954

CRYSTAL OSC ILLATOR C I RCUIT Filed Feb. 13, 1943 NoN-osc|LLAmeOSCILLATING REGION REGION I G J 5 D a Zg s'mnrms U k REGION H d C e a L5 a m I! a u w a O z Fig.2.

CONTROL ELECTRODQ CURRENT Ld (D E c g E 2, a FREQUENCY 5 s O i! aInventory Regmald L.Downey,

Aral/ewe) HI Patented Feb. 20, 1945 CRYSTAL OSCILLATOR CIRCUIT Reginald.L. Downey, Scotia, N.: assiznorto General: Electric Company, acorporation of New York Application February 13, 1943, SerlalNo.475,732.

3v Claims.

My invention relates to oscillator circuitsv employing piezoelectricdevices.

In oscillator circuits employing piezoelectric devices and especiallyinv circuits employing an electron discharge device having a tuned anodecircuit and having a crystal connected between the control electrode andcathode, it has been necessary to tune th anode circuit appreciably onthe high frequency side of the operating frequency of the piezoelectricdevice in order .to obtain proper conditions for quick and stablestarting of the oscillator. After oscillations have been started, theanode tuned circuit has been retuned for higher output. In such a case,however, if

oscillations are halted for any reasons whatso- 1 ever, oscillations maynot start again unless the circuit is again tuned to. the high frequencyside of the operating frequency. For high frequency crystals, such asthose having a natural frequency in the range of 1,000 to 10,000kilocycles, the optimum operation of the oscillator is usually obtainedwhen the anode oscillatory circuit is tuned to about 102 per cent to110' per cent of the crystal frequency, depending on the output,frequency stability and tube current conditions. For low frequencycrystals, as for example, in the range ing of oscillations even thoughthe anode tuned circuit is tuned to a frequency providing optimumoperating conditions.

In accordance with my invention there is provided in combination with apiezoelectric or crystal oscillator having a tuned anode circuit,automatic means dependent upon the oscillator output for tuning thecircuit between resonance at a frequency at which the crystal oscillatesreadily and at another frequency at which the desired output is obtaineddepending upon whether the oscillator is in non-oscillating oroscillating condition. In other words, the anode oscillatory circuit ischosen toibe resonant at a frequency different from and preferablyhigher than the frequency of the crystal and also higher than thefrequency at which optimum operation is had after starting is effectedbut at which frequency the generation of oscillations readily starts.Means is also provided which is responsive to the starting ofoscillations for automatically tuning the oscillatory circuit to aresonant, frequency which more closely approaches the frequency at whichoptimum operation is had. Thus in the absense of control electrode biasthe anode circuit tuning is increased progressively farther on the highfrequency side toward an optimum starting condition. This means maycomprise a reactance device arranged to tune the anode circuit oi acrystal oscillator. The reactance device is controlled from thegridcurren-t ofthe crystal oscillator such a manner that anode tankcircuit conditions are optimum for starting when the crystal is notoscillating and: optimum to. produce maximum output voltage when thecrystal is oscillating.

The features of my invention which I believe to'be novel are set forthwith particularity in'the appended claims. its organization and mannerof operation, to: gether with further objects and advantages thereof maybest be understood by reference to thefole lowing description taken inconnection with the. accompanying drawing in which Fig. 1 .i'slaschematic. diagram of an oscillator circuit. embodying the principles ofmy invention, and. Figs. 2. and .3. illustrate operating characteristicsor the circuit shown in Fig. 1.

In Fig. 1 there is shown an electron discharge oscillator circuit l0comprising-an electron discharge device ll havin an anode, 11, a.control electrode 13, and a cathode Hand having a crystal connectedbetween arid, itemdeathude 1 land a tuned circuit connected between the.anode l2 and the cathode M. The oscillator control electrode is biasedby eans o r sistors 18: and '20. A n d n d ir it- 5 comprising. anoscillatory circuit including an inductance 16 and :a variable capacitorI1 is; connected between: the. anode, l2 and the cathode l4. through-asource of potential 40 and ground. oscillatory circuit 1515 completedfrom the anode. l2. to the. cathode: for radio frequency currents .bymeans of. .a. suitable kbypass condenser 2|. Radio frequency in the gridcircuit is bypassed around the resistor ill by a suitable condenser 22.The cathode is grounded.

- In Fig. '2 there are illustrated typical characteristics of the.oscillatorcircuitshown-inFig. 1. In

Fig. 2a, variations of anode current of the elec- My invention itself,both. as to decreased from a maximum, as by varying the variablecapacitor l1, oscillations begin at point a and the anode currentfollows the solid line to the point D where oscillations cease. Theanode current rapidly rises to c where the current remains constant atlower frequencies as indicated by the line cd. If the condenser is thentuned from the low frequency end toward the high frequency end,oscillations will not begin again until the point e is reached, at whichpoint the anode current will suddenly drop to J on the original curve.If the tuning is continued in the high frequency direction, the anodecurrent will follow the curve toward a. If, however, the tuning isreversed so that the condenser is tuned in the direction of 15 lowerfrequency, the anode current goes back along the curve toward b. Thearea defined by the letters I), c, e, f, is a region of instability. In

thi range of frequencies, the crystal oscillator will not begin tooscillate if the resonant frequency of the tuned circuit is lower thanthe frequency represented by the letter f on the curve of I 2c althoughoscillations continue, if once started. Therefore, if it is desired toassurecertain starting, the oscillatory circuit must be tunable to apoint between a and f on the anode current characteristic. Starting isprogressively better as the frequency is increased so that the optimumstarting point is well out toward the high frequency end of the curve,as at the point 9,

for example.

On the other hand, it is desired to operate the oscillator circuit at afrequency at which maximum output is obtained. In Fig. 2c there isillustrated the variation in oscillator output, plotted as ordinates,against frequency, plotted as abscissa. It is to be noted that thefrequencyjs plotted on the same scale in each of Figs. 2a and 20, sothat direct comparison can be made between these figures. shows thatwhile starting conditions are optimum at a point such as g in Fig. 2a,at that frequency the output is very low. For maximum output it will beseen that a frequencycorresponding to the point It is desirable. It istobe noted that this 45 point is in the unstable zone where starting isuncertain.

-Means has been provided for automatically shifting thetuning of .theoscillatory circuit between-frequencies at which optimum startingconditions prevail and at which optimum operating conditions are had.

Connected across the anode tuned. circuit is a form of inductance typereactance tube circuit including a phase shifting circuit comprising theseries connected inductance and the resistance 26. There is also shown ablocking capacitor 2! in the series circuit including inductance 21having acapacity such that it has low impedance to the oscillationsproduced. Across the phase shifting circuit there is disposed anelectron discharge device preferably of the pentode type comprising ananode 3|, a cathode 32, control electrodes 33, 34 and 35. The anode 3iis connected to the pper end of the inductance 25, the control electrode33. is .connected to a point between the capacitor 21 and the resistor28, the electrode 34 is connected to the positiveterminal of a suitablesource of potential 36, the electrode is connected to the cathode andthe cathode 32 is connected to ground through the biasing. resistor 31shunted by the capacitor 38. The lowerend of the resistor 28 isconnected to ground for alternating. currents by means of a suitablecondenserdl.

5 age of the tuned circuit l5.

Comparison of Figs. 2a and 2c The reactance of the winding 25 and theresistance of the resistor 26 are chosen so that the radio frequencyvoltage on the electrode 33 is approximately 90 out of phase with theoutput volt- Under such conditions, the discharge device 30 drawscurrent from the oscillatory circuit which is 90 lagging, thereby givingthe effect of an inductance of approximate value L 1 Rgm where L0 is theeffective inductance of the reactance circuit, L is the inductance ofthe coil 25, R is the resistance of the resistor 26 and gm is the mutualconductance of the electron discharge device 30, i. e'., the rate ofchange of plate current with respect to changes in control electrodevoltage.

With the circuit as shown in Fig. 1, if oscillations cease, as when thetransmission period for the transmitter with which the oscillator isassociated is ended, the oscillator output ceases, resulting in minimumbias of the control electrode 25 33 of the electron discharge device 30.Therefore,

the gm of the discharge device increases and L0 is decreased. Therefore,the effective reactance of the tuned oscillatory circuit i5 is decreasedand the resonant frequency of the tuned circuit is increased under theassumed conditions. The variation of resonant frequency of the tunedcircuit with the voltage on control electrode 33 is shown in Fig. 3 inwhich frequency is plotted along the abscissa (to the same scale as inFig. 2) and the 35 control electrode voltage is plotted along theordinates. Accordingly, the oscillator is automatically tuned to afrequency such as represented by the point gin Figs. 2 and 3.

After oscillations start, as when the oscillator is again switched on,the reverse action takes place. The eifective inductance is increased sothat the reactance of the tuned circuit is decreased and accordingly theresonant frequency of the anode circuitis changed to correspond with thepoint It, for example. In other words, the current in discharge device30 i phased to -simulate a reactance of sign and. magnitude to effectthe desired operation.

The operating and starting points forthe oscillator circuit depend uponthe bias of the electron discharge device 30 including the'resistances18 and 31. In order to provide control and enable the operator to selectthe operating and starting frequencies, one of these resistances is made5 variable. Accordingly, the resistor l8'is of the conditions isrelatively wide as seen in Fig. 2a.

In Fig. 2b there is plotted control electrode current for the tube I asordinates, against frequency of the tuned anode circuit l5. Maximumcurrent occurs near the operating frequency of the crystal l 9.

The inductance values of the windings l6 and 25 are made as nearly equalas practicable but large enough to prevent undue loadingof the outputcircuit by the resistanceii fi. .Use of the 7-5; resistor 31 andcondenser 38,.in 'theLcathode' clr'-.

cuit of the device 30 stabilizes operation and prevents excessivecurrent being drawn by the discharge device 30. The capacitor'M not onlybypasses the radio frequency current to ground but ma also beproportioned to effect a time delay in the operation of the circuit ifsuch is desired.

Tubes of relatively high gm are necessary for the reactance tube orelectron discharge device 38. It has been found that four type 1852tubes connected in parallel work satisfactorily and with such anarrangement it was possible to operate a crystal ground to oscillate at100 kilocycles at the point of maximum grid current and even on the lowfrequency side of this point at approximately maximum output and stillachieve instantaneous starting.

A. capacity-resistance type phase sp itting circuit may be employed, ifdesired, instead of the inductance-resistance type circuit.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from my invention in itsbroader aspects, and I, therefore, aim in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. The combination, in a crystal oscillator, of an electron, dischargedevice having an anode, a cathode, and a control electrode, apiezoelectric crystal connected between said control electrode andcathode, an oscillatory circuit connected between said anode andcathode, said oscillatory circuit being resonant at a frequency higherthan the frequency of said crystal and higher than the frequency atwhich optimum operation is had after starting is effected but at whichfrequency the generation of oscillations readily starts, and meansresponsive to oscillations in said oscillatory circuit for lowering theresonant frequency of said circuit to a frequency substantially moreclosely approaching said frequency at which optimum operation is had.

2. The combination, in a crystal oscillator, of an electron dischargedevice having an anode, a cathode, and a control electrode, a crystalconnected between said control electrode and cathode, an oscillatorycircuit connected between said anode and said cathode, said oscillatorycircuit being tuned to a frequency at which the generation ofoscillations under the control of said crystal readily starts butdifferent from that at which optimum operation of said oscillator i hadafter the generation of oscillations starts, and means responsive to thegenerated oscillations to maintain said circuit-tuned. to a frequencysubstantially more closely approximating said frequency at which optimumoperation is had and upon cessation of oscillation to retune saidcircuit to said frequency at which oscillations more readily start,whereby both ready starting and optimum operation thereafter is had.

3. The combination, in a crystal oscillator, of an electron dischargedevice having an anode, a cathode, and a control electrode, a crystalcon,- nected between said control electrode and cathode, an oscillatorycircuit connected between said anode andsaid cathode, said oscillatorycircuit being tuned to a frequency at which the generation ofoscillations under the control of said crystal readily starts butdifferent from that at which optimum operation of said oscillator is hadafter the generation of oscillations starts, and means device tosimulate a reactance of sign and magnitude to maintain said oscillatorycircuit tuned to said last mentioned frequency.

4. In combination, an oscillator of the type having an electrondischarge device having an anode, a cathode and a control electrode, apiezoelectric device connected between the control electrode and thecathode, and an oscillatory circuit connected between the anode andcathode, and means dependent upon the output of said oscillator forautomatically tuning said circuit between resonance at a frequency atwhich said device readily begins to oscillate and at another frequencyat which a desired output is obtained according to whether saidpiezoelectric device is in non-oscillating or oscillating condition,respectively. i

5. In combination, an oscillator of the type having an electrondischarge device having an anode, a cathode and a control electrode, apiezo-' electric device connected between the control electrode and thecathode, and an oscillatory circuit connected between the anode andcathode, and means in shunt to said oscillatory circuit for varying theeffective reactance of said circuit in accordance with the output ofsaid oscillator thereby to vary the resonant frequency thereof betweenfrequencies at which said device readily begins to oscillate and atwhich a desired output is obtained, according to whether saidpiezoelectric device is in non-oscillating or oscillating condition.

6. In combination, an oscillator of the type having an electrondischarge device having an anode, a cathode and a control electrode, apiezoelectric device connected between the control electrode and thecathode, and an oscillatory circuit connected between the anode andcathode, and a phase shifting circuit in shunt with said oscillatorycircuit and operative in accordance with the output of said oscillatorto tune said oscillatory circuit between resonance at a frequency atwhich said device readily begins to oscillate and at another frequencyat which a desired output is obtained according to whether saidpiezoelectric device is in a non-oscillating or an oscillatingcondition, respectively.

7. In combination, an oscillator of the type having an electrondischarge .device having an anode, a cathode and a control electrode, apiezoelectric device connected between the control electrode and thecathode, and an oscillatory circuit connected between the anode andcathode,

and means dependent upon the output of said oscillator for automaticallytuning said circuit between resonance at a frequency at which saiddevice readily begins to oscillate and at another frequency at which adesired output is obtained according to whether said piezoelectricdevice is in non-oscillating or oscillating condition, respectively, andmeans for adjusting the. point of operation for the desired output ofsaid oscillator.

8. In combination, an oscillator of the type having an electrondischarge device including an anode, a cathode and a control electrode,a piezoelectric device connected between the control electrode and thecathode, resistance means in shunt with said piezoelectric device forproviding bias for the'control electrode of said discharge device, anoscillatory circuit connected between the anode and cathode of saidelectron discharge device, and means dependent upon the output of saidoscillator for automatically tuning said circuit between resonance at afrequency at which said device will readily begin to oscillate andanother frequency at which substantially maximum output is obtainedaccording to

